Reusability and stability of a novel ternary (Co–Cd–Fe)-LDH/PbI2 photoelectrocatalytst for solar hydrogen production

The design of highly active and cost-effective photoelectrocatalysts for effective hydrogen generation becomes a mandatory issue due to the demands on sustainable solar fuels. Herein a novel ternary Co–Cd–Fe LDH/PbI2 nanocomposite (T-LDH/PbI2NC) was fabricated by combining strategies of doping and in-situ loading of ternary Co–Cd–Fe LDH. The morphological, structural, and optical properties of PbI2, T-LDH, and T-LDH/PbI2 NC were studied by different techniques. LDH narrows the bandgap of the nanocomposite to 2.53 eV which prolongs the lifetime of the photo-induced electrons. Subsequently, the use of T-LDH/PbI2 NC improves the photoelectrocatalytic (PEC) H2 production rate. T-LDH/PbI2 NC shows a catalytic H2 production rate of 107.53 mmol h−1 cm−2 with IPCE% of 83.8% for 307 nm and 67.3% for 508 nm. The ABPE% reaches its supreme of 4.24% for − 0.58 V and 5.41% for − 0.97 V, these values are the highest values yet for LDH-based photocatalysts. The influences of the operating temperature and monochromatic illumination on the PEC performance were studied. Also, the electrochemical surface area, thermodynamic parameters, and Tafe slopes are calculated to label the hydrogen evolution mechanism. Moreover, the stability and reusability of the T-LDH/PbI2 NC photoelectrode were investigated. This work not only illustrated a simplistic and accessible way to produce a new category of highly efficient photocatalysts compared to the previously reported LDH-based PEC catalysts but also demonstrates a new point of view for improving PEC performance towards industrial water splitting under sunlight irradiation.

Increasing demands for energy and elevating the environmental crisis have inspired researchers to develop low-cost, environmentally friendly, and reasonable sources of energy. Water splitting through the photoelectrochemical (PEC) route is considered as one of the promising approaches to produce hydrogen as chemical fuel 1,2 . The PEC water splitting process needs semiconductor photocatalysts to convert sunlight photons directly to hydrogen molecules as clean fuels. The semiconducting materials require a remarkable performance in sunlight absorption, electrons-holes separation, and electrons/holes mobility. But still, the electron-hole recombination is the main challenge in the choice of photocatalyst for PEC 3,4 . Layered-metal halides (for example; PbI 2 and CdI 2 ) are concerned with increasing interest in electrocatalysis because of their uses in the design of perovskite halides. These perovskite structures offered noticeable photoelectrocatalytic performances 5 PbI 2 offers high photoelectrocatalytic performances among the different SC materials however it necessitates illumination with photons of wavelengths less than ∼ 350 nm (absorption band onset). It has a bandgap wider than 3.10 eV. Then, its performance under visible light is limited. As a result, the modification of PbI 2 using co-catalysts with suitable bandgaps for the visible light photons is considered the most common method to improve its photocatalytic hydrogen evolution (PHE) efficiency 6,7 .
Layered double hydroxides (LDH) have attracted much attention because of their high layer charge density along with two-dimensional interlayer spaces of height, which are available for generating a rational path for charge conveying 8 . NiFe, ZnCr, CoAl-LDH, and NiAl-LDH were used as co-catalysts in the field of water splitting to generate O 2 and H 2 9-14 . Zhang et al. has designed a modular catalyst of Ni-MgO-Al 2 O 3 via the template of NiMgAl-LDH. He showed excellent coke-and wintering-resistance in the drying of methane reaction 15 . Kulamani et al. 16 has reported that NiFe-LDH/g-C 3 N 4 photocatalyst shows excellent photoelectrocatalytic performances
The mean crystallites sizes (D C ) of PbI 2 were estimated utilizing Scherrer's relation. The calculated mean crystallites size was ~ 70 nm 24 . The mean value of the microstrain for PbI 2 was ~ 0.2%. The dislocations density (δ d = N/D C 2 , N is constant) was also estimated to evaluate the density of defects and the quality of the crystal. The smallest δ d for PbI 2 was calculated when N = 1 25 . The obtained value of δ d is 2.05 × 10 −4 that refers to the high quality of the synthesized PbI 2 crystal .
The XRD chart of T-LDH/PbI 2 illustrates the main XRD peaks of the (Co-Cd-Fe)LDH but with observed shifts in the position (Fig. 1C). Also, a mixed-phase between PbI 2 and LDH appears in this pattern. The observed XRD peaks are referred to diffractions from (003), (006), (101), (012), (009), (107), (018), and (113) planes. Moreover, the significant rise in the XRD peaks intensities refers to the distribution of PbI 2 on LDH layers and the good distribution of PbI 2 between the layers of LDH.
Morphologies of the samples. The morphologies of (Co-Cd-Fe)LDH and T-LDH/PbI 2 NC were investigated using FE-SEM and TEM (Fig. 2). The prepared (Co-Cd-Fe)LDH looks like agglomerated nanoparticles stacked together, Fig. 2A. These particles have non-uniform shapes with different sizes. These particles are subsequently folded as our brains. A close examination of the sample using a high magnification SEM image reveals the existence of many small nano protrusions on the surface of LDH particles. After the formation of PbI 2 on Co-Cd-Fe LDH, PbI 2 distributes between the layers of LDH particles and increases the porosity of LDH. As a result, the surface area has increased and the particle size distribution was found to be 70 ± 10 nm, Fig. 2B. A TEM micrograph of T-LDH/PbI 2 NC, Fig. 2C, illustrates the presence of PbI 2 particles on Co-Cd-Fe LDH particles. It is seen that the layered structure of the LDH prevents the agglomeration of the PbI 2 particles. This is useful for the separation of photogenerated electrons and holes. Hence, it is highly expected that this photocatalyst can be applied efficiently for photoelectrochemical hydrogen generation. The nanoporous features of the nanocomposite are shown in the magnified images of     www.nature.com/scientificreports/ Figure 3A-C shows high resolution-transmission electron microscopy (HR-TEM) images of T-LDH with PbI 2 . The distributions of PbI 2 particles on the Co-Cd-Fe-LDH platelets are clearly observed. The highly magnified HR-TEM images were shown in Fig. 3B,C are used to confirm the fine structure of T-LDH/PbI 2 NC, which showed the stacking of the layered nanosheets. The Co-Cd-Fe LDH component showed a plate-like morphology. The selected area electron diffraction (SAED) pattern illustrates the existence of the diffraction rings, inset of Fig. 3A, these rings confirmed the polycrystalline state and homogeneous distribution of PbI 2 on the Co-Cd-Fe LDH layers. These results may enhance the ECSA-value and improve the separation of interfacial charge transfer between Co-Cd-Fe-LDH and PbI 2 particles.
The EDX spectrum, Fig. 3D, of T-LDH/PbI 2, and the inserted quantitative analysis Table in Fig. 3 clearly indicate the presence of cobalt, iron, and cadmium signal within the walls. The molar ratios of Co:Cd:Fe were found to be approximately 1:1:1. These ratios are in good agreement with the ratios used during the preparation of T-LDH.   Figure 4B confirms the presence of strong interactions between Pb-I clusters. Symmetric and asymmetric modes are observed at 3055 and 3700 cm −1 of the Pb-I 28 .
The peak at 2400 cm −1 was ascribed to the water stretching region. After the combination, the FTIR spectrum of T-LDH/PbI 2 NC exhibits a shift to lower wavenumbers (redshift) which may be ascribed to the distribution of Pb-I 2 into layers of LDH was shown at Fig. 4C.
Optical properties of Co-Cd-Fe LDH, PbI 2 , and T-LDH/PbI 2 NC. The light absorption property of all samples is explored by UV-Vis absorption spectra as displayed in Fig. 5. The absorption edge of pure PbI 2 appears at 300 nm. No important features are observed in the Vis/IR regions. This was due to its intrinsic bandgap absorption Fig. 5. After the growth of T-LDH/PbI 2 NC, the absorption edge of Co-Cd-Fe LDH redshifts to the visibleregion. So, a noticeable improvement in absorption can be observed in Fig. 5. Also, the layers of LDH facilitate the motion of the photo-produced electrons 29 . As well, the T-LDH/PbI 2 NC showed a wider absorption band in the visible-light-region. These enhanced absorption capabilities result from the extension of the band to cover a broad region of the incident photons (300-800 nm). This range represents > 43% of sunlight at the Earth's sur-   wherever β , ℓ , and C p are the material density, quartz cell width (1 cm), and suspended material concentration.
The bandgap energies were determined by inferring the linear portion of (α A E ph ) 2 -E ph plot with the E ph -axis, Fig. 6.
The bandgap energy is estimated to be 3.04 eV for PbI 2 , which agreed to the previously stated bandgap values for nanostructure PbI 2 (> 2 eV). This bandgap is originated from Pb s to Pb p interband transitions was shown in Fig. 6A. Also, there are two other bands at 2.31 and 2.51 eV due to the existence of two discrete absorption plasmons as a result of the quantum confinement effects and the orbital hybridizations between the I p-orbitals and Pb s-orbitals . On the other hand, the bandgap values are estimated from to be 2. 28    The current density using the T-LDH/PbI 2 NC photoelectrode generated the greater no of electrons than others under white light exposure. This is ascribed to its highly electrical surface charges and the suitable optical bandgap which in turn to increase in the absorbance in the Vis/IR range. In pristine PbI 2 , there are decline in kinetics of water oxidation on the surface thus in valence band, the accumulation of positive holes will occur and stimulate electron/hole recombination in the conduction band, which displays low photo response 33 .
While Co-Cd-Fe LDH (TLDH) represented as an effective co-catalyst. It distinguishes with highly conductivity and faster carrier transfer which enhances the kinetic of water oxidation so helps in initiation of the removal of the photogenerated holes accumulated at the surface of system 34 .
While after introduction of Co-Cd-Fe LDH with PbI 2 , the performance of ternary T-LDH/PbI 2 towards PEC is higher than others. Its outstanding efficiency is attributed to the synergistic effect of TLDH and PbI 2 in the ternary T-LDH/PbI 2 .
Generally, the white light illumination has a crucial effect on PEC technique.the light stimulates the electrons of photocatalyst for hydrogen generation than dark conditions. The photocurrent was J ph = 53.27 and 3.12 eV mA cm −2 for dark condition as shown in Fig. 6. This amazing increase is assigned to the photoexcitation of electrode and generation of charged carrier (e − /hole + pair) which helps in water splitting and hydrogen generation 35 .
Tafel slopes and ECSAs of the photoelectrodes. Tafel curves, Voltage-log(J ph ), are presented in Fig. 8A-C from the J ph -Voltage characteristics of Fig. 8A to mark the hydrogen evaluation reaction (HER) mechanism 36 . The Tafel slopes of the straight lines in Fig. 8B,C are represented by β1 and β2 at low and high HER potential 36 . β 1 and β 2 values are reported in Table 1 with their standard deviations and the correlation R 2 -coefficients. Tafel slopes of 30, 40 and 120 mV dec −1 apply to Volmer-Tafel (recombination is rate-limiting) mechanism, Volmer-Heyrovsky (PEC desorption is rate-limiting) mechanism, and the dependency on different reaction paths of surface coverage by adsorbed H 2 . β 1 and β 2 remind us of the over-potentials required to increase the HER rate by 10 folds 36 . Thus, the calculated β 1 and β 2 , Table 1, of the T-LDH/PbI 2 NC electrode (49.91 and 79.61 mV dec −1 ) evidenced its improved PEC characteristic in HER.
The values of ECSAs for the three electrodes are obtained using the Randles-Sevcik equation, ECSA = I(R T/v D) 1/2 /[0.446 (C n F) 3/2 ], where n = 1 refers to one electron contribution in the redox reaction, F and R denote to the Faraday and gas-molar constants 37 . Also, C signifies the analytes concentration, T signifies the reaction temperature, and D represents the analyte-diffusion constant 37 . Using J-V curves, Fig. 8, the ECSAs values for the three electrodes are calculated using ECSA = Q·(m·C) −1 . Whereas Q, m, and C refer to the hydrogen-adsorption charges in the negative-scan after double-layered charges correction, photocatalyst mass, and the complete    The stability of TLDH/PbI 2 NC was further confirmed using the EDX analysis after ten runs at Fig. 10. The chemical composition of T-LDH/PbI 2 did not change which confirms the stability of photocatalyst after many runs for H 2 generation. But the oxygen content in the catalyst matrix was decreased from 12.55 to 10.92, which may be referred to as the generation of oxygen vacancies due to the applied potential in the presence of KOH electrolyte. Oxygen vacancies (OVs) are considered one of the defects formed in the semiconductors. These defects are generated by the removal of an oxygen atom from the catalyst matrix while it is still charged with extra electrons. OVs which diffuse at the interfaces layer in the LDH and form an interlayer of a different crystal phase due to their influence on the phase stability 40 .
The XRD analysis after ten runs can be considered as another proof of the stability of T-LDH/PbI 2 NC was shown at Fig. 1D. The structural properties of T-LDH/PbI 2 are almost the same after many runs. The phase of the catalyst did not change which was attributed to the stability of photocatalyst after many runs for H 2 generation. But the diffraction peak (003) disappears after ten runs for H 2 generation which confirms the formation of oxygen vacancies. In these studies, the enhanced performance of PEC was systematically correlated with a higher density of OVs, which cause a higher Incident Photon to Current Efficiency (IPCE) 40. Effect of optical filters and calculation of conversion efficiencies. Different wave length filters (307-636 nm) were applied for determination of the most suitable wave length for photoelectrode in PEC system, was represented at Fig. 11A. The PEC behaviors in 0.3 M KOH (100 ml) solution at 25 °C and increment 1 mV/at different optical filters were studied. We noticed The alteration of the monochromatic light changes the J ph value. The behavior of the T-LDH/PbI 2 NC photoelectrode under the monochromatic illumination can be significant related to its absorbance response for different wave lengths light and its ability to absorb a large part of visible sunlight.
Generally, IPCE (incident-photon-to-current conversion efficacy) and ABPE (applied bias-photon-to-current efficacy) are main factors for qualifying the PEC solar hydrogen generation. IPCE value was measured at different wave length filters for determining the actual number of charge-carriers that related to the generated photocurrent per incident photon. Using the power density (P (mW cm −2 )) and the wavelength (λ (nm)) of the monochromatic light as shown Fig. 11B, the IPCE is given using Eq. (4) 43 IPCE values in − 1 V in different wave lengths is presented in Fig. 11B. The maximum IPCE for T-LDH/PbI 2 NC photoelectrode was ~ 83% for 307 nm. Another maximum of ~ 67% was observed at 508 nm. The positional wavelengths of the two maxima are matched well with the absorption edges observed in the optical analysis of T-LDH/PbI 2 NC, Figs. 5 and 6.
However, ABPE value describes the photo-response efficiency of a T-LDH/PbI 2 NC electrode under an applied voltage 44,45 . ABPE can be calculated using the following equation: This result may be due to the appearance of two band gaps and can be described based on the well-known photoelectric effect. This photoelectrode has a higher efficiency than the previously reported photoelectrodes 46 . The full 3D data of ABPE vs. the applied potentials and the incident wavelengths for the two maxima are presented in the color fill contours, Fig. 11D,E. The noticeable electrode response at lower potential can be advantageous for PEC cells.
Electrochemical impedance spectroscopy (EIS). Charge carrier dynamics play a vital role in the photocatalytic water-splitting process in deciding the photocatalytic performance of photoelectrode. To investigate the charge carrier dynamics of the T-LDH/PbI 2 NC electrode, EIS data have been measured by an electrochemical workstation (CHI660E) at room temperature. The photoelectrode was immersed in a 0.3 M KOH electrolyte and the EIS measurements were carried out under illumination at 0 V (vs Ag/AgCl) for a frequency range of 0.01-100,000 Hz. For this photoelectrode, the Nyquist plot is shown in Fig. 12A. This plot exhibited a semicircle at high frequencies due to charge transfer processes in electrode/electrolyte boundaries (charge transfer resistance) and two straight line segments observed at low frequencies with slopes ~ 44° and ~ 69° due to diffusion-controlled processes (Warburg impedance) and additional minimal capacitive activity (double-layer capacitance) as shown in the insets of Fig. 12A. That is to say, mixed diffusion and kinetic controlled routes are illustrated by the EIS data. The results obtained are fitted to a simple equivalent circuit in order to explain the EIS measurements through the hydrogen evolution process. Figure 12B inset displays the suggested Randle equivalent circuit for the simulation of EIS results using the ZSimpWin software (version 3.2; https ://echem -softw are-zsimp win.softw are.infor mer.com/3.2/). This circuit contains the electrolyte resistance (R s = 22 Ω with Fitting error = 0.04965) that can be obtained from Nyquist plot intercept at high frequency, charge transfer resistance (R ct = 4.3 Ω) equals to the semicircle diameter in the Nyquist plot, double-layer capacitance (C dI = 1.472 μF) and Warburg impedance (W = 9.525 × 10 -5 ). The reported Rs and R ct values are much smaller than any literature values for LDH-based electrodes, which promoting the PEC hydrogen production [47][48][49] . Figure 12B,C presents Bode plots for the T-LDH/PbI 2 NC electrode, measured at room temperature using 0.3 M KOH electrolyte at 0 V (vs Ag/AgCl). Figure 12B illustrates the total impedance (Z) vs. the frequency, whilst Fig. 12 displays the behavior of the phase vs. the logarithm of the frequency and shows a resistive regime related to the R ct at low frequency as well as capacitive contributions related to the C dl of the electrode at high frequencies 50 . From Fig. 12C, the maximum phase shift (Ө max in degree), and the frequency at the maximum phase (f max in Hz), are estimated to be 40.9° in 0.022 Hz. The lifetime of the charge carriers can be estimated www.nature.com/scientificreports/ from Fig. 12 via the relationship τ n = 1/2π ƒ max 50 . The value of the obtained lifetime of the charge carriers for the T-LDH/PbI 2 NC electrode is estimated to be 7.23 s. The obtained parameters indicate a great reduction in the charge recombination at the electrolyte/electrode interfaces. This also refers to a kinetically facile PEC system, improved ionic conductivity, and electrolytes diffusion through the T-LDH/PbI 2 NC electrode. Therefore, this photoelectrode showed the highest photocatalytic performance to produce large amounts of H 2 compared to the previously reported LDH-based electrodes.
Effect of applied temperature and calculation of thermodynamic parameters. The operating temperature is considered a vital parameter that can affect the photoelectrode performance. Figure 13A shows the influence of the applied temperature from 298 to 358 K on the performance of the T-LDH/PbI 2 NC photoelectrode. The J ph  www.nature.com/scientificreports/ increases with increasing the applied temperature to reach its maximum value (73.8 mA cm −2 ) at 253 K. Then, the hydrogen generation rate is increased sharply (1.5 fold) by increasing temperature. i.e., a high reaction temperature will improve dehydrogenation kinetics and release hydrogen at elevated temperature. This increase is due to the decline of the photoelectrode bandgap energy and the increase of the charge transfer rate. According to the Arrhenius plots 50 (Fig. 13B), the apparent activation energy (Ea) of the HER using T-LDH/PbI 2 NC photoelectrode was calculated to be 9.09 kJ mol −1 . This value is lower than most of the previously reported values for other LDH-based catalysts 51 . Also, the other thermodynamic parameters such as enthalpy (ΔH*) and entropy (ΔS*) were estimated using the Eyring equation, Fig. 13C. The ΔH* of T-LDH/PbI 2 NC is deliberated from the slope to be 12.99 kJ mol −1 . While from the intercept, the ΔS* for T-LDH/PbI 2 NC is 78.97 kJ mol −1 . Table 2 illustrates the PEC performance of our T-LDH/PbI 2 NC photoelectrode comparing with the previously studied LDH-based PEC catalysts [52][53][54][55][56][57][58] . As shown in this Finally, a simple hydrogen generation mechanism is illustrated in Fig. 14 and presented as follows. The incorporation of PbI 2 nano-semiconductor with a second nano-semiconductor of lower bandgap (Co-Cd-Fe LDH) form a promising photocatalyst to harvest visible light. Some photocatalytic composites can enhance the PEC properties because of the overlapping between the band gaps of two different photocatalysts, which could favor the charge carrier transfer and separation. During applying the external potential bias, photons excite electrons and holes' separation. The excited electrons migrate from the valence-band (VB) to the conductionband (CB) of LDH. Then they are transferred to the PbI 2 catalyst. After that, the transferred electron reacts with the adsorbed H + ion-producing H 2 molecule. Simultaneously, the residual holes are combined with and removed by the sacrificial reagents of the KOH solution, whereas PbI 2 avoids the eh + recombination 52 . Finally, PbI 2 facilities the transfer of the additional CB electrons to Pt and hurries the generation of H 2 at the active Pt surfaces as shown in Fig. 14.

Experimental details
Preparation of photocatalysts. Preparation of PbI 2 electrode. Precipitate PbI 2 by dissolving both lead (II) nitrate and potassium iodide in distilled water then add a few drops of potassium iodide to the lead (II). As soon as the solutions touch, bright yellow lead iodide is produced. Finally, add the rest of the potassium iodide Fabrication of the PEC photoelectrodes. Three different photoelectrodes are fabricated to be used for the PEC hydrogen production. 3% of each photocatalyst (PbI 2 , Co-Cd-Fe LDH, and T-LDH/PbI 2 NC) is mixed with 3% of C 6 H 4 (CO 2 C 4 H 9 ) 2 plasticizer (DBP, 99.8%) and 3% of (C 2 H 3 Cl) n (PVC, 99.8%) in the least amount of (CH 2 ) 4 O (THF, 99.9%). DBP, PVC, and THF were obtained from the Egyptian Middle East company. The three products of the mixing process are moved to three 5 cm-Petri dishes. The mass of each batch is 0.35 g. Then, the three Petri dishes are left to dry and sealed off with three filter papers. By fixing the amount of THF and carrying out the drying process for one day, the thickness of each photoelectrode is fixed to be 200 μm.
Characterization of different photocatalysts. The XRD patterns of PbI 2, Co-Cd-Fe LDH, and T-LDH/ PbI 2 NC were obtained by Philips X ' Pert 1 -MRD X-ray diffraction (λ CuKa = 0.15418 nm). Samples morphology is investigated using a field-emission scanning electron microscope (FESEM,HRTEM, Zeiss SUPRA/55VP with GEMINI/column). (Fourier Transform Infrared Spectroscopy (FTIR) was performed by A Shimadzu-FTIR-340-Jasco spectrometer to obtain the important functional groups of the samples. Finally, the optical absorbance behaviors of the products are investigated by Lambda 900-UV/Vis/IR Perkin Elmer spectrophotometer up to 1200 nm.

Conclusion
In summary, a novel technique for loading Cd-Co-Fe-LDH/PbI 2 has been introduced to fabricate an efficient nanocomposite photocatalyst. For comparison, the different properties of PbI 2 , Co-Cd-Fe LDH, and T-LDH/ PbI 2 NC were investigated using various instruments; XRD, FTIR, HR-TEM, FE-SEM, and UV-Vis-IR spectrophotometer. The growth of LDH on PbI 2 prevents the agglomeration of LDH nanoparticles and allows the distribution of the particles to increase the surface area and decrease the particle size. Loading of LDH narrows the bandgap of PbI 2 from 3.04 to 2.53 eV for T-LDH/PbI 2 NC, which prolongs the lifetime of the photoinduced electrons. Consequently, the application of T-LDH/PbI 2 NC improves the PEC H 2 production rate to reach 107.53 mmol h −1 cm −2 and IPCE% to reach 83.8% in 307 nm and 67.3% in 508 nm. The ABPE% reach its maximum value (4.24%) at − 0.58 V and (5.41%) at − 0.97 V. To the best of our knowledge, the performance of T-LDH@PbI 2 NC as a PEC catalyst is higher than any previously reported LDH-based photocatalysts. The effects of the operating temperature and monochromatic illumination on the PEC performance were studied. Also, the electrochemical surface area, thermodynamic parameters, and Tafel slopes are calculated to label the hydrogen evolution mechanism. The T-LDH/PbI 2 NC photoelectrode displayed lower Tafel slopes and a much higher electrochemical surface area compared to T-LDH and PbI 2 electrodes. Moreover, the activation energy of T-LDH/PbI 2 NC was 9.09 kJ mol −1 , which was lower than any previously reported value for LDH catalysts. This study has provided a new viewpoint to design highly active photocatalysts for solar light-driven H 2 production.